1. Field of the Invention
The present invention relates to an ultrasonic diagnosing apparatus having a function of visually displaying function information associated with the movement of tissues of a living body, such as blood flow data, by an ultrasonic Doppler method of obtaining movement data of an object by using the Doppler effect of an ultrasonic wave.
2. Description of the Related Art
In ultrasonic diagnosing, diagnosis is performed on the basis of various information obtained by using an ultrasonic wave, e.g., anatomical information, typically a B-mode image, motion information of an organ of a living body, typically an M-mode image, and function information associated with the movement of tissues of a living body, which is obtained by using the Doppler effect, typically blood flow imaging.
When the above-described ultrasonic diagnosis is to be performed, a living body is scanned with an ultrasonic beam either by an electronic scanning method or by a mechanical scanning method. In the electronic scanning method, an ultrasonic beam is scanned by electronically controlling the drive timings of a plurality of transducer elements and/or the processing timings of echo signals received by a plurality of transducer elements. In the mechanical scanning method, an ultrasonic beam is scanned by mechanically moving an ultrasonic transducer. Electronic scanning will be described in detail below.
Electronic scanning is normally performed by using an ultrasonic transducer array in which a plurality of ultrasonic transducer elements are properly arranged.
In linear electronic scanning, a predetermined number of transducer elements of the above-mentioned transducer elements are used as a group, and one transmission/reception operation of an ultrasonic beam is performed by using this group. In this method, transducer elements selected as the group are sequentially switched to slide the transmission/reception position of an ultrasonic wave. For example, the transmission/reception position of an ultrasonic wave is electrically shifted by sequentially shifting/selecting transducer elements of the group one by one. In addition, the excitation timings and/or reception signal timings of elements of the selected transducer elements, which are respectively located at central and peripheral portions of the beam, are shifted from each other so as to focus the ultrasonic wave by using a difference in phase between sound waves generated by the respective transducer elements or the reception signals. This operation is called electronic focusing. An ultrasonic wave received by ultrasonic transducer elements, i.e., an ultrasonic wave reflected by tissues of a living body (ultrasonic echo), is subjected to predetermined processing as an electrical signal, and an image based on ultrasonic echo data, e.g., a tomographic image, is formed to be displayed on a TV (television) monitor or the like.
When sector electronic scanning is to be performed, the above-mentioned plurality of ultrasonic transducer elements are used as a group as a whole, the excitation timings and/or reception signal timings of the respective transducer elements are sequentially changed to sequentially steer an ultrasonic beam to be transmitted/received and to scan a sector-like region. The subsequent signal processing is basically the same as that in the above-described linear electronic scanning.
In addition to the above-described linear and sector electronic scanning methods, a combination or modification of these methods is employed to perform electronic scanning.
In the mechanical scanning method, an ultrasonic transducer is attached to a scanning mechanism, and the ultrasonic transducer is mechanically driven by the scanning mechanism so a to scan an ultrasonic beam.
B- and M-mode methods are typical ultrasonic imaging methods used for diagnosis. The B-mode method is used to obtain a B-mode image as a tomographic image by mixing signals obtained by transmission/reception of an ultrasonic wave. The M-mode method is used to obtain an M-mode image associated with changes in echo data over time, which is obtained by stationary scanning in the same beam direction. An M-mode image represents changes in position of an ultrasonic wave reflecting portion over time, and is suitably used to diagnose a moving organ such as a heart.
An ultrasonic Doppler method, typically blood flow imaging, is employed to visualize function information of a living body by obtaining data based on the movement of moving tissues in the living body. This method will be described in detail below.
In the ultrasonic Doppler method, movement data of a moving object is obtained by using the ultrasonic Doppler effect in which when an ultrasonic wave is reflected by the moving object, the frequency of the reflected wave is shifted in accordance with the moving velocity of the object. More specifically, an ultrasonic pulse having a predetermined frequency is transmitted into a living body, and a frequency shift based on the Doppler effect is obtained from changes in phase of the reflected wave (echo), thus obtaining data associated with the movement of the moving object as a reflection source of the echo.
According to the ultrasonic Doppler method, the directions of blood flows at various positions in a living body can be determined together with the states of the blood flows, e.g., whether the blood flows are disturbed or normal.
A conventional ultrasonic diagnosing apparatus capable of performing blood flow imaging based on the above-described ultrasonic Doppler method and normal B- and M-mode imaging will be described below with reference to FIG. 1.
In order to obtain blood flow data from an ultrasonic echo, an ultrasonic transducer 1 of, e.g., an array type is driven by a transmitter 2 so as to repeatedly transmit an ultrasonic pulse at a predetermined pulse rate frequency by a predetermined number of times in a given direction (beam direction). The ultrasonic wave is reflected by scattering tissues moving in a living body, blood cells in this case, and returns as an ultrasonic echo which underwent a Doppler shift (fr+.DELTA.fr). This ultrasonic echo is received by a receiver 3 through the ultrasonic transducer 1, and is phase-detected by a phase detector 4, thereby obtaining a signal consisting of phase data of only .DELTA.fr, i.e., a Doppler signal and a clutter component as an unnecessary low-frequency component. This signal is converted into a digital signal by an A/D (analog-to-digital) converter 5. The clutter component is then removed by an MTI filter constituted by a digital filter in an MTI 6 for performing MTI (moving target indicator) processing. In the MTI 6, a Doppler shift signal based on a blood flow is frequency-analyzed by a high-speed frequency analyzer, which employs an autocorrelation system for obtaining a color Doppler image in a real-time manner. As a result, Doppler data is obtained which includes moving direction (blood flow direction) data, moving velocity (blood flow velocity) data, velocity turbulence data, and Doppler signal power data. The Doppler data is written in a frame memory of a digital scan converter (DSC) 7. An image memory 8 stores data of a plurality of frames, as needed, and is used to perform cinematographic loop reproduction, in which these data are sequentially and repeatedly read out to be reproduced and displayed.
The Doppler data such as the power, turbulence, and velocity data read out from the frame memory of the DSC 7 are input to an RGB conversion section 9a in synchronicity with a horizontal sync signal from a display system (H synchronization).
The Doppler data are then converted into R (red), G (green), and B (blue) signals by the RGB conversion section 9a in response to a control signal El from a host CPU (central processing unit) 20a. As shown in FIG. 2, the RGB conversion section 9a comprises a color bar generator 11, multiplexers 12a and 12b, and an RGB conversion circuit 13. Doppler data B input from the DSC 7 to the RGB conversion section 9a is mixed with color bar data from the color bar generator 11 by the multiplexer 12a. This composite data is further mixed with B-mode data C from a B/W (black and white) processor 21 in synchronicity with the time phase thereof. The color bar data is data for displaying a color bar on the screen of a display 10 as a reference scale of color display. The B- or M-mode data C from the B/W processor 21 is image data consisting of an envelope-detected echo signal for displaying a B- or M-mode image on the screen of the display 10. In this case, scanning of an ultrasonic wave for the B or M mode including transmission/reception of the ultrasonic wave for the above-described Doppler method is performed, and the B/W processor 21 forms a B- or M-mode image from a signal obtained by performing envelope detection of a reception echo signal obtained by the receiver 3, and outputs it as black/white (B/W) image data C. Because it is apparent that the B/W processor 21 also includes a DSC and a cinematographic loop reproduction image memory, though they are not shown.
The data output from the multiplexer 12b is converted into an RGB signal D by the RGB conversion circuit 13 and is supplied to the monitor display 10.
In this manner, the color data based on the Doppler data such as the blood flow and velocity data is mixed with the B/W data for a B- or M-mode image, which is obtained by another system, by using the RGB conversion circuit 13. As a result, the composite data is displayed on the monitor 10 as, e.g., a two-dimensional blood flow velocity image, which is obtained by superposing the above-mentioned Doppler data on the B/W image such as the B- or M-mode image.
In the above-described conventional ultrasonic diagnosing apparatus, a Doppler shift component is uniquely converted into a color data signal in such a manner that a Doppler shift caused by the movement of the object in a direction to approach the ultrasonic transducer 1 is displayed in red, and a Doppler shift caused by the movement of the object in a direction to move away from the ultrasonic transducer 1 is displayed in blue.
In anatomy and the like, arteries and veins are normally indicated by red and blue, respectively. If the display colors of Doppler shifts coincide with this coloring, no problems are posed. However, if the display colors of Doppler shifts differ from the general display colors some confusion may be caused.
For example, when a carotid artery is to be clinically diagnosed, a Doppler signal effectively used for diagnosis is sometimes obtained only when the ultrasonic transducer is brought into contact with a neck portion of a patient so as to direct it to a head portion. In this case, since the display colors of Doppler shift components are uniquely determined, arteries and veins are respectively displayed in blue and red, contrary to the general classification by color. In such a case, an operator, e.g., a doctor, who is familiar with the general display colors, i.e., red representing arteries and blue representing veins, may confuse arteries with veins.